p-Hydroxyacetophenones by Selective Solubilization

Ind. Eng. Chem. Res. 2002, 41, 1335-1343

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Separation of o-/p-Hydroxyacetophenones by Selective Solubilization and Sorption on Weak Base Ion-Exchange Resins Vilas G. Gaikar* and Hyacinth M. Anasthas Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India

A two-stage process has been developed for the separation of o-/p-hydroxyacetophenones (o-/pHAP), both of which are intermediates for the pharmaceutical industry. The first step is the selective solubilization in a hydrocarbon solvent; o-HAP dissolves well in heptane and toluene, but p-HAP does not. The intermolecular hydrogen bonding among p-HAP molecules makes it almost insoluble in heptane and has solubility in toluene as low as 0.03 mol/kg of solvent. The first step gives a hydrocarbon solution of o-HAP with a very small amount of dissolved p-HAP, while pure p-HAP precipitates as a solid in high yield (∼90%). The traces of p-HAP from the organic solution can be removed in the second step using its selective sorption in weakly basic resins, giving a solution with o-HAP. The sorption of p-HAP on the resins by acid-base interactions between the resin’s amino group and p-HAP’s acidic hydroxy group is aided by the unfavorable solvation of p-HAP in the organic phase. Introduction Acylation of a phenol followed by Fries rearrangement gives a mixture of o- and p-hydroxyacetophenones (HAPs) (C8H7O2). The composition of the product varies depending upon conditions of the reaction and catalyst.1 Being intermediates in the pharmaceutical industry, pure HAPs have significant industrial importance. Their separation is conventionally carried out by crystallization or fractional distillation under reduced pressure or by steam distillation, as o-HAP is steam volatile. An improved and more efficient method for the resolution of these isomers will definitely be useful at the industrial level. Apart from the differences in their physical characteristics, the difference in the acidic strengths of HAPs can be exploited for their separation. As compared to o-HAP (pKa ) 10.22), p-HAP (pKa ) 8.05) is a stronger acid by 2 orders of magnitude and would react preferentially with a base of appropriate strength. For instance, in the process of dissociation extraction, which exploits the differences in dissociation constants (or pKa values) and distribution coefficients, a neutralizing agent is used in stoichiometric deficiency to react preferentially with the stronger component of a mixture.2 Several such reactive techniques have been reviewed by Gaikar and Sharma.2 Although highly selective, these methods suffer from the disadvantages of a net consumption of chemicals and the problem of disposal of waste streams generated during the recovery step. Jagirdar3,4 had exploited the difference in solubilities of nitro-substituted compounds in organic solvents, such as toluene, to separate o-/p-nitrophenols and o-/pnitroanilines. The ortho isomer in both cases is more organic soluble than its para counterpart. The complete separation of o/p isomers was not, however, possible by selective solubilization alone, and multistage reactive extraction using aqueous NaOH or aqueous HCl, as the case may be, had to be used after the first step to remove * To whom correspondence may be addressed. Phone: 9122-4145616. Fax: 91-22-4145614. E-mail: [email protected].

a few percent of p isomer dissolved in the organic solvent. Equilibrium separations using neutralizing agents cannot give complete removal of the p isomer, and the removal of its traces from the organic solutions becomes increasingly difficult as its concentration drops to lower values. Adsorptive techniques have a potential for separation of these close boiling point compounds, particularly when the component to be removed is present in trace quantities. Separation of alkylphenols and removal of acidic impurities from organic solvents by adsorption on commercial weakly basic ion-exchange resins have been reported recently with excellent capacity and selectivity.5,6 A combination of selective solubilization with reactive adsorption on functionalized polymers may provide an attractive alternative for the separation of o-/p-HAPs. For the separation of phenols, a basic resin can function as the mass separating agent. The neutralizing reaction between the acidic phenol and amino group on the polymeric resin should provide the separation principle. Ion-exchange resins are commercially available in different forms, and the flexibility of tailoring them for specific needs by manipulating the functional group is advantageous for their application as separating agents. The regeneration of the weakly basic resins by a polar solvent, such as methanol or acetone, is a feasible option obviating the waste disposal problem commonly associated with the reactive techniques. If the primary adsorption is conducted from an inert hydrocarbon solvent, then the adsorbed solute can be easily washed off from the resin by a polar solvent. The net consumption of energy will then be decided by the efficiency of recovery methods for these solvents. The selective solubilization is explored in this work as the first step for the separation of HAPs because of the intra- and intermolecular hydrogen bonding in o-/ p-HAPs, respectively. It is also expected to modify the sorption characteristics of HAPs in the second step of the separation. The expected interaction is between the lone pair of electrons on nitrogen of the amino group of the resin and the acidic hydrogen of phenol. In the case of o-HAP, because of intramolecular H bonding, the

10.1021/ie010471+ CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002

-OH group is less likely to interact with the resin giving a very high sorption selectivity toward p-HAP. No proton transfer is expected between the interacting species because of the nonpolar nature of the solvent used in these studies.

Table 1. Solubility Profile of p-HAP in Different Solvents solubility of p-HAP (mol/kg solvent)

solvent

Materials and Experimental Procedures S. D. Fine Chemicals, Mumbai, India, supplied o-/phydroxyacetophenones, heptane, toluene, dichloromethane, and methanol. The weak anion-exchange resin (Indion 850) and the strong anion-exchange resin (Indion 810) were obtained from Ion Exchange Ltd., Mumbai. The other weak anion-exchange resin (Tulsion A-8X MP) was obtained from Thermax Ltd., Pune, India. The choice of these two IERs (Indion 850 and Tulsion A-8X MP, having similar structures) was made to check the reproducibility of the results obtained using similar type of resins produced by different manufacturers. All three resins have a polystyrene backbone crosslinked with divinylbenzene (7-8% cross-linking). Indion 850 and Tulsion A-8X MP are weakly basic macroporous resins with tertiary amine as the functional group and are not truly ion-exchange resins. The resins have surface area of 35 × 103 and 23 × 103 m2/kg and average pore size of 30 and 24 nm, respectively. Indion 810 resin is a strongly basic macroporous resin with quaternary ammonium groups with an OH- counterion and has surface area of 27 × 103 m2/kg and a pore size of 27 nm, as per the data sheets provided by the manufacturers. The resins were first washed with a 5% (w/v) aqueous solution of sodium hydroxide, and the excess alkali was removed by washing the resins with a large volume of distilled water. The resins were then thoroughly washed with methanol, then dried in an oven for 6 h at 333 K, and subsequently cooled to the room temperature of 303 K. Solubilization Studies. Because o-HAP is highly soluble in most organic solvents, the solubility experiments were restricted to p-HAP, which shows negligible to sparingly soluble behavior in the selected solvents. A known volume of organic solvent was taken in a stirred vessel to which a known amount of p-HAP was added. The suspension was then kept in a constanttemperature bath (at 303 K) and was stirred for about 2 h using a magnetic stirrer. A period of 30 min was usually sufficient for equilibration as established from independent studies, but to ensure equilibrium, the stirring was continued up to 2 h. The saturated solution was then filtered to remove the undissolved solute, and the solution was analyzed by UV spectroscopy for the single component studies [λmax, p-HAP ) 279 nm and λmax, o-HAP ) 254 nm] using a Chemito 2100 scanning UVvis spectrometer. The solubility in toluene was determined by a weight-loss method. The weight of the undissolved p-HAP was measured after separation from the saturated solution by filtration. The repeated runs showed that the method is quite accurate and gave reproducibility within 1.5%. For such a low solubility of p-HAP, this deviation was quite acceptable. For the solubility studies of p-HAP in the presence of o-HAP, a known weight of p-HAP was added into an organic solution containing o-HAP at a known concentration. Because o-HAP was completely miscible with all of the solvents, there was no possibility of it separating out of the solution because of the dissolution of p isomer nor was there any appreciable partitioning of

heptane toluene dichloromethane methanol a

activity coefficient of p-HAP

in the in the single presence single presence component of o-HAPa component of o-HAPa insoluble 0.028 0.784 10.353

0.010 0.109 0.849 11.517

386.5 17.5 4

1094 118.7 15.7 3.8

Concentration of o-HAP ) 1.0 mol/dm3; temperature: 303 K.

o-HAP into solid p-HAP phase, as shown by the further experiments during the selective solubilization step where the precipitated solid contained no o-HAP at all. The suspension was stirred with a magnetic stirrer for 2 h, and then the residual solid was separated from the solution by filtration. The saturated solution was analyzed by high-performance liquid chromatography (HPLC) on a TOSOH-CCPE chromatograph equipped with a UV-8100 detector and using a C-18 reversedphase column. The mobile phase was a 80:20 (v/v) MeOH/H2O mixture. The accuracy of the HPLC unit is very high, and we repeatedly checked the results because of extraordinarily high selectivity observed during the experiments. Adsorption Using Ion-Exchange Resins. The equilibrium adsorption studies were carried out in a specially fabricated glass cell to avoid loss of the solvent during the adsorption process.7 A known amount of the adsorbent (1 g) was taken in the cell, and the solutions of HAP in selected solvent (10 cm3) with varying concentrations were added into the cells. The cells were then kept in a constant temperature water bath to attain equilibrium with occasional shaking. The volume changes of the resin because of sorption of the solvent/ solute into the polymeric phase were noted separately for each experiment by noting the expanded volume of the solid bed in graduated glass tubes and also by microscopically observing individual swollen resin beads. The adsorption was continued for a period of at least 4 h, although it was checked by separate experiments initially that a time period of 30 min was sufficient to reach the equilibrium in the stirred conditions. After the equilibrium was reached, the solution was separated from the resins, and residual concentration of the phenols in the solutions was determined by HPLC, as described previously. The adsorbed concentration was obtained from material balance. The experiments were also repeated for individual and mixtures of HAPs in different organic solvents and using different resins. Results and Discussion Selective Solubilization in Different Organic Solvents. The solubility of p-HAP was determined in four different organic solvents: heptane, toluene, dichloromethane, and methanol. Heptane is a completely nonpolar solvent without specific interaction with any of the HAPs. Toluene, as an aromatic solvent, shows slightly better polarity because of polarizability of the aromatic ring. Dichloromethane (DCM) and methanol are both polar solvents and interact better with the phenols, particularly methanol, which is a good H-bond donor as well as an acceptor because of an -OH group. The solubility values of both HAPs are reported in Table 1. o-HAP is highly soluble in heptane and toluene, while

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1337

p-HAP is almost insoluble in these two solvents. The difference in the solubilities is very evident in heptane in which pure p-HAP is practically insoluble. o-HAP, by virtue of its structure, shows intramolecular hydrogen bonding, while p-HAP shows intermolecular hydrogen bonding. Nonpolar hydrocarbon solvents, like heptane, cannot easily break down the intermolecular hydrogen bonding in p-HAP. Interaction with such hydrocarbons involves only London dispersion forces that are not capable of overcoming specific H bonding among the p-HAP molecules. o-HAP, on the other hand, has intramolecular hydrogen bonding and dissolves into heptane with a relative ease, as individual molecules can be solvated because of their poor cohesive forces. In toluene, which is a polarizable solvent because of the π electron cloud, the difference in solubilities of the isomers is still high but is reduced from that in heptane. In the case of a more polar solvent like DCM, the difference in the solubilities is reduced substantially as compared to heptane and toluene. Methanol, being itself a hydrogen-bond donor as well an acceptor, takes part in hydrogen bonding with the functional groups of HAPs and can solubilize both isomers to the larger extent but mostly as individual species unless the concentration of phenols is very high.

Because the maximum difference in the solubility behavior of HAPs was observed in heptane, it was selected for selective solubilization of o-HAP from its mixtures with p-HAP. The solubility of p-HAP in the presence of o-HAP, however, increased slightly from that of pure p-HAP. The increase, in the case of mixtures, is due to the presence of o-HAP in the solutions with which p-HAP can interact. The solutions of HAPs thus exhibit a nonideal behavior due to strong intermolecular interactions. This behavior also should have an impact on the sorption behavior from the solutions, as indeed are the observations as described later in this work. The activity coefficient (γ) of HAP in the solutions is the measure of its interaction with a solvent. Poor solvation should give γ far greater than unity and very low solubility of the solute in the solution phase. If γ has a smaller value, then the solubility of solute should increase. No data are, however, available for estimation of the activity coefficients of these isomers, because the available group contribution methods, such as UNIFAC,8,9 cannot distinguish between them. Because the solubility of p-HAP is relatively low, its activity coefficient can be approximated as the inverse of its mole fraction at saturation (i.e., γ ) 1/x). Table 1 reports activity coefficients of p-HAP in different sol-

Table 2. Separation of HAPs by Selective Solubilizationa feed mixture

solid residue

o-HAP (g)

p-HAP (g)

heptane (dm3)

2.68 3.6 35.0

0.22 0.34 1.75

0.05 0.05 0.50

a

o-HAP (g)

solution phase

p-HAP (g)

o-HAP (g)

p-HAP (g)

0.176 0.28 1.567

2.676 3.57 35

0.044 0.060 0.183

Temperature: 303 K.

vents in the order γheptane > γtoluene > γDCM > γmethanol. The solvation is the best in methanol, as indicated by the lowest activity coefficient of p-HAP in the methanol. Because p-HAP is practically insoluble in n-heptane, its concentration could not be measured either by HPLC or UV spectroscopy, which are very sensitive methods. Approximate estimates of the activity coefficient of p-HAP, in the presence of o-HAP at an appreciable concentration, are also reported in Table 1. The solubility values of p-HAP in the presence of o-HAP are not substantially different in polar solvents from those observed in the absence of o-HAP, indicating that interaction between two isomers is not strong in polar solvents. Probably, individual HAP molecules are solvated independently in these solvents because of the solvent’s own strong polar nature and, in the case of methanol, because of its ability to take part in H bonding. Nevertheless, if the concentration of phenols is increased, it should affect the solution nature and can give rise to the nonideal behavior of the solutions. In the nonpolar solvents, however, the relative increase in the solubility of p-HAP in the presence of o-HAP is appreciable. The mutual interaction effects are, therefore, more important in nonpolar solvents. The solubility difference of HAPs in heptane can be exploited for the separation of product mixtures obtained from a manufacturing process. If o-HAP is present in a large percentage in the mixture, it may cosolubilize some amount of p-HAP into the heptane phase, reducing the separation efficiency. However, if the product consists of a p-HAP rich mixture, the separation efficiency should be very high. The results of selective solubilization of o-HAP from the mixtures of HAPs are shown in Table 2. The feed mixture of HAPs was in a liquid state because of the higher concentration of o-HAP. As soon as the mixture was added into heptane, o-HAP dissolved into the solution, and simultaneously, p-HAP precipitated as a crystalline product with almost 85-90% yield. The solid product was washed with heptane to remove o-HAP adhering to the surface of the solid, if any. In all cases, p-HAP so obtained was 100% pure, as shown by its UV spectrum and also by the HPLC analysis. The very high solubility of o-HAP and almost insoluble nature of p-HAP in heptane ensures a clean separation of the mixture. The purity of o-HAP in the solution phase was in the range of 95-98%. A small amount of p-HAP, however, dissolved into the solution because of o-HAP. It should be possible to remove p-HAP from the heptane solutions by adsorption on functionalized resins. Because p-HAP is a stronger acid, it was expected that the weakly basic resin group should preferentially neutralize it. The extension of the pKa concept from aqueous solutions to organic nonpolar conditions may not be directly possible because of the minimal degree of dissociation of phenols in these solvents. Adsorptive Separation Using Ion-Exchange Resins. The equilibrium studies were conducted first for a

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Figure 1. Sorption of o-hydroxyacetophenone on weak base resins from heptane (temperature: 303 K).

Figure 2. Sorption of o-HAP (filled symbols) and p-HAP (hollow symbols) on weak base resins from toluene (temperature: 303 K).

range of concentration of HAPs to collect sorption data on three basic resins: Indion 850, Tulsion A-8X MP, and Indion 810 at 303 K. The solvents used were the same as those employed for the solubilization studies. Figures 1-4 show that HAPs are significantly sorbed on the resins from organic solutions at all liquid-phase concentrations. The sorption data for p-HAP from heptane could not be obtained because p-HAP is practically insoluble in heptane. The sorption of both HAPs was maximum on Indion 850, followed by Tulsion A-8X MP, and was the least on Indion 810. o-HAP showed the maximum sorption from heptane (∼0.73 mol/kg), followed by that from methanol (∼0.57 mol/kg), then from toluene (∼0.42 mol/ kg), and the least from DCM (∼0.35 mol/kg). The uptake of p-HAP was the maximum from toluene, even at low solution concentrations, and the least from methanol. By analogy, the uptake of p-HAP from heptane by these resins must be the highest. The solvent apathy alone would drive p-HAP out of the solution onto the adsorbent surface. p-HAP was sorbed to a large extent from toluene and DCM in comparison with o-HAP, while both HAPs were sorbed to the similar extent from methanol. The uptake

Figure 3. Sorption of o-HAP (filled symbols) and p-HAP (hollow symbols) on weak base resins from dichloromethane (temperature: 303 K).

Figure 4. Sorption of o-HAP (filled symbols) and p-HAP (hollow symbols) on weak base resins from methanol (temperature: 303 K).

of p-HAP from DCM is almost 14 times greater in the lower concentration range than the uptake of o-HAP at the same liquid-phase concentration. Preferential uptake of p-HAP, in comparison with o-HAP, is clearly evident from these studies. The uptake of the phenols by the weak-base resins can be considered in terms of the Lewis acid-base interaction between the phenol and amino group of the resin.10,11 The nitrogen of the tertiary amino functional group of the resin carries a lone pair of electrons. Although sterically hindered by two methyl groups, nitrogen can form a hydrogen bond with the acidic hydrogen of the phenolic species. In solvents such as heptane, it is very unlikely that phenolic species would dissociate into ions to give rise to ionic species for ionic interaction. The strength of such an interaction would be dependent on the charges on the phenolic H and amino N, their ability to come close enough to form the hydrogen bond, and also on the solvent. o-HAP, being intramolecularly chelated, has not only a lower charge on its hydrogen, but it is not easily available for any interaction with the resin, resulting in its lower uptake. On the other hand, the acidic hydrogen of p-HAP can freely form a hydrogen-bonded complex with


nitrogen of the resin structure, resulting in a substantial increase in its uptake. As compared to the weakly basic resins, the strongly basic quaternary resin (Indion 810) showed lower capacities as well as relatively poorer interaction with phenols. In solvents, such as heptane, replacement of the OH- counterion of the resin structure by the weakly acidic phenol is unlikely. In fact, the lower dielectric constant of heptane would make the ionic interaction much stronger between the charged centers than the ion-dipole interaction expected between the charged quaternary group and the phenolic dipole. If, however, phenol is converted into phenate by OH- ion, one can expect a higher affinity of phenols toward the resin. The experimental results indicate otherwise, even in polar solvents where such dissociation may be present. The uptake of both HAPs by resins from methanol is approximately to the same extent on individual resins, but the difference in two HAPs sorption by the resins appreciably reduced. The preferential solvation of both HAPs in methanol can eliminate the intramolecular hydrogen bond in o-HAP and the intermolecular hydrogen bonding between p-HAP molecules. The sorption of HAPs takes place from methanol, therefore, as independent entities unaffected by intra/intermolecular hydrogen bonding of HAPs. Strong solvation of HAPs by methanol itself should reduce their adsorption tendency. A certain amount of solvent also penetrates into the polymeric network of the resin depending upon the affinity of the solvent for polymer structure. In fact, the swelling of the resin may become a prerequisite for the sorption. The polymeric structure of the ion-exchange resins swells significantly when brought in contact with organic solvents because of the penetration of organic solvents into the polymeric network of resin phase. The swelling ratio (i.e., ratio of volume of the swollen resin to that of a dry resin) was found to be dependent on the nature of the solvent. It was the least with heptane (1.03-1.05), followed by toluene (1.05-1.17), DCM (1.08-1.24), and the maximum in methanol (1.17-1.30). The swelling of a resin depends on its interaction with solvent, which was least favored in inert solvents and was maximum in polar solvents. The swelling in the presence HAPs differed considerably from that in the presence of pure solvents and was different for two HAPs. The swelling ratio in the presence of p-HAP (1.35) was marginally higher than that in the presence of o-HAP (1.26). The difference in the swelling shown by resins in the presence of HAPs from that in their absence indicates contribution of the phenols to the swelling of resins. The tendency of the resins to swell was higher for Indion 850 (probably due to more number of exchange sites) than for Tulsion A-8X MP, which also accounts for the difference in the solute uptakes by these two otherwise similar resins. The swelling of the resins makes more volume available in the resin phase, and particularly, the adsorption sites are more accessible to the solute. The final sorption capacity of the resin for the phenols was much less than the equivalent exchange capacity of the resin. Although the swelling of the resins was supposed to provide access to the exchange sites well inside the resin beads, the much lower capacity indicated either an inaccessibility of those sites inside the resin phase or probably a molecule occupying more than one sites. The steric hindrance to the interaction between the phenol and

Figure 5. Inverse selectivity of solvent sorption versus o-HAP concentration (solvent: heptane).

Figure 6. Inverse selectivity of solvent sorption versus HAP concentration (solvent: toluene; o-HAP (filled symbols) and p-HAP (hollow symbols)).

resin site also cannot be ruled out because of the size of molecules and cross-linking in the resin structure. Heptane does not have the ability to interact with the polymer, being inert, while toluene, because of its π electron cloud, interacts slightly better, but the interaction of a polar solvent like methanol is the strongest. The uptake of solvents during the sorption of HAPs on Indion 850 were computed from the swelling measurements and are compared in Figures 5-8 as inverse selectivity for the solvent for different solvents and resins. The solvent uptake was higher during the sorption from polar solvents and for Indion 850 as compared to the other resins. This difference in the swelling of the resins in nonaqueous conditions depends on degree of cross-linking, porosity of the resins, and the nature of solvent. The uptake of the solvents was in the following order: heptane < toluene < DCM < methanol. The uptakes of methanol and DCM were almost twice that of nonpolar solvents. The inverse selectivity values are the lowest in heptane indicating that the sorption of HAPs is the best from heptane. The selectivity toward p-HAP was higher in comparison with o-HAP in all cases, which increased further with the increase in HAP concentration.

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Figure 7. Inverse selectivity of solvent sorption versus HAP concentration (solvent: dicholoromethane; o-HAP (filled symbols) and p-HAP (hollow symbols)).

Figure 8. Inverse selectivity of solvent sorption versus HAP concentration (solvent: methanol; o-HAP (filled symbols) and p-HAP (hollow symbols)).

Absorbed Versus Adsorbed Amounts. If it is assumed that the additional volume available because of swelling of the resin is occupied by the solution having the same concentration as in the external liquid phase, then the amount of HAPs actually associated with the resin polymeric structure or the functional sites can be estimated. It is clear that a major part of the solute is taken up by adsorption, with a small fraction of solute being taken up by absorption in the swollen resin. The relative amounts of solute present in the resin as ab/ adsorbed were, therefore, quantified for both isomers. Figures 9 and 10 show plots of the absorbed HAP versus the adsorbed HAP from toluene on different resins. The other solvents gave a similar trend. The absorbed o-HAP exceeded the adsorbed o-HAP for Indion 850 at higher concentrations, while for the other resins it was somewhat similar, whereas for p-HAP, the absorbed amounts were almost nil when compared with the adsorbed amounts. The adsorbed p-HAP was at least four times greater than adsorbed o-HAP at saturation, indicating preferential sorption of p-HAP by the sites. The adsorbed amounts of HAPs were fitted into the Langmuir’s isotherm. Table 3 gives the fitted values of

Figure 9. Adsorbed (hollow symbols) and absorbed (filled symbols) o-HAP from toluene on different resins.

Figure 10. Adsorbed (hollow symbols) and absorbed (filled symbols) p-HAP from toluene on different resins. Table 3. Parameter Values of Langmuir Isotherm of HAPs solvent heptane resin

K

Γ∞

toluene K

Γ∞

o-HAP Indion 850 26.07 0.75 5.03 0.29 Tulsion A-8X MP 14.80 0.63 3.38 0.19 Indion 810 12.38 0.54 2.52 0.16 Indion 850 Tulsion A-8X MP Indion 810

a

a

a

a

a

a

DCM K

Γ∞

methanol K

Γ∞

2.99 0.29 10.37 0.40 2.52 0.20 4.68 0.26 2.05 0.14 2.85 0.25

p-HAP 442.8 0.81 55.23 1.88 368.0 0.71 41.44 1.07 318.8 0.41 26.27 0.72

5.32 0.49 1.12 0.38 0.55 0.28

a

Could not be measured because of the insolubility of p-HAP in heptane. K is given in dm3/mol; Γ∞ is given in mol/kg.

the isotherm parameters. The Langmuir adsorption constant (K) is the maximum for Indion 850, followed by Tulsion A-8X MP, and the least for Indion 810 for both HAPs. The adsorption constant (K) was the maximum for adsorption from heptane and toluene and the least for sorption from polar solvents. This behavior was expected from the activity coefficients of HAPs in different solvents. As discussed earlier, the adsorption constant of p-HAP from the heptane solution must be

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1341 Table 4. Modified Equilibrium Constants (K′) for p-HAP solvent toluene dichloromethane methanol

resin

K′ (dm3/mol)

Indion 850 Tulsion A-8X MP Indion 810 Indion 850 Tulsion A-8X MP Indion 810 Indion 850 Tulsion A-8X MP Indion 810

1.15 0.95 0.83 3.15 2.36 1.50 1.32 0.28 0.14

still higher than that obtained with toluene as the solvent. Also, the equilibrium constant is the highest for p-HAP and Indion 850 resin in comparison with o-HAP from all organic solvents except methanol, indicating that the interaction of p-HAP with the resin is relatively much stronger. In methanol, the trend is reversed, with o-HAP showing a slightly higher K value than that of p-HAP. The polar nature of methanol results in the breaking of an intramolecular hydrogen bonding of o-HAP, which will thus become available for the formation of hydrogen-bonded complexes with the resin. p-HAP also loses its intermolecular hydrogen bonding on solvation by methanol and, being more polar, is preferentially solvated in methanol. It may be possible to include the activity coefficient of a solute to consider, as an approximation, the solvent effects in the adsorption. For example, the concentration terms in the Langmuir equation, if are replaced by the product of activity coefficient and concentration, then the K′ should represent truly the interaction between the resin and the solute. The modified Langmuir isotherm is of the form

Γ K'γC ) ∞ 1 + K'γC Γ

(1)

If the adsorption is solely influenced by solvation of solute in the solution phase, then K′ values in a modified isotherm equation should be the same or at least similar in two different solvents. The equilibrium constants were modified for p-HAP by incorporating its activity coefficient in the modified Langmuir equation and are reported in Table 4. The trend in the K values remained the same, with the difference in their magnitudes decreasing substantially. The values are not, however, very close. The nonideality present in the adsorbed phase was neglected in the aforesaid analysis and was not considered the multimolecular adsorption because of stronger intermolecular interaction possible among these phenols, particularly during adsorption from nonpolar solvents. The adsorption of p-HAP is influenced strongly by nonideality in the organic phase, which is reinforced by specific interaction between the resin and p-HAP. A solvent showing poor affinity for p-HAP forces p-HAP out of the solution and promotes its sorption on a solid surface, in this case, in the resin phase. The adsorption of HAPs seems to be driven by its rejection by the solvent and not as much by its interaction with the basic functional group of the resin. It should be interesting to investigate the adsorption of these phenols on nonfunctionalized styrene-divinylbenzene copolymers from the same solvents to identify the role the functional group plays. Hydrogen bonding between nonfunctionalized polymers such as styrene-divinylbenzene copoly-

Figure 11. Selective sorption of p-HAP (hollow symbols) against sorption of o-HAP (filled symbols) on Indion 850 resin from different solvents.

mers and phenolic species (and amines) has been reported in the literature.10,11 Because we did not find any sorption of HAPs on acidic resins with the same polymeric structure, the role of simple dispersion forces seems to be insignificant as compared to the acid-base interaction between the phenol and amino group. In the present studies, it is obvious that the amino group does play a significant role in adsorption. Mixture Studies. From the single component studies, the difference in the sorption affinities of o-/p-HAPs toward different resins was evident. Because the difference was maximum on Indion 850 resin, the separation of HAP mixtures was attempted using the same. Figure 11 shows the uptake of HAPs from their mixtures in different solvents on Indion 850. The uptake of p-HAP is very high in comparison with o-HAP from dichloromethane and toluene, while the uptake from methanol is the least. In toluene and DCM, the sorption of p-HAP increased with the increase in concentration, but in methanol, the sorption of p-HAP, although higher than that of o-HAP, decreased with the increase in p-HAP concentration. Significantly, however, sorption of o-HAP increased with its concentration in the methanol phase, where because of the high concentrations of both phenols, the nature of the solution should have been substantially altered. Figure 12 indicates the mole fractions of p-HAP in both the adsorbed and liquid phases on a solvent-free basis. From methanol, the mole fraction of p-HAP in the adsorbed phase decreased, indicating that the selectivity toward p-HAP decreased at higher concentrations. Increased interactions of the HAPs with each other at higher concentrations should be responsible for these observations. Figure 13 gives the separation factor of p-HAP with respect to o-HAP on a solvent-free basis. The separation factor is the highest in toluene, followed by dichloromethane, and the least for methanol. This indicates that the separation of the isomers will be easier in toluene as compared to the other two solvents. Heptane is, however, the best solvent. It is able to dissolve o-HAP completely in the first stage, giving almost pure p-HAP, and whatever traces of dissolved p-HAP, because of o-HAP, are removed completely from the solution on

1342


Figure 12. Mole fraction of p-HAP in adsorbed state versus mole fraction of p-HAP in solution on solvent-free basis (resin: Indion 850). Figure 14. A schematic of two-stage separation process for separation of o-/p-HAPs.

Figure 13. Separation factor as function of o-HAP concentration (resin: Indion 850).

contact with ion-exchange resin, giving complete separation of HAPs. Optimized Two-Step Process. An alternative process for the separation of these isomers was arrived at on the basis of these results. The first step involves the selective solubilization of o-HAP in heptane. Because this process can reduce the quantity of p-HAP in the heptane solution to trace amounts, a significant degree of separation can be achieved at this stage. This step finally gives pure p-HAP in good yields, but traces of p-HAP remain in the heptane solution. The second step involves the removal of trace amounts of p-HAP remaining in the organic phase by preferential sorption on basic ion-exchange resins and giving pure o-HAP in solution. The typical block diagram for the process is given in Figure 14. In an experiment conducted on a larger scale, the mixture used for the preferential solubilization contained 35 g of o-HAP and 1.75 g of p-HAP. The volume of heptane added was 500 cm3. After selective solubilization of o-HAP, the suspension was filtered to separate precipitated p-HAP from the organic phase. p-HAP was washed with heptane (55 cm3) to recover

1.57 g (89.5% recovery) in pure form (100% purity). The extract solution phase contained 0.183 g of p-HAP and 35 g of o-HAP. The extract solution phase was then pumped through a column (2.0 cm diameter and 37 cm height) filled with the Indion 850 ion-exchange resin. For the complete passage of the extract solution phase (555 cm3), no p-HAP was detected in the effluent solution. The recovery of o-HAP in the effluent solution was 78.5% (100% purity). The adsorbed phenols were completely desorbed when 220 cm3 of methanol was passed through the column. The major advantages of this method are good recovery of both isomers with very high purity in a two-stage operation, which would be impossible to attain in a single-stage distillation or steam distillation. Also, the solvent can be recovered and recycled without significant losses of the components. Another advantage lies in the overall significantly lower energy requirement than physical separation involving vacuum distillation/ steam distillation, as the process needs vaporization of volatile solvents with low heats of vaporization. This method should be equally applicable for the separation of similar mixtures, such as o-/p-nitrophenols, o-/p-nitroanilines, o-/p-phenylenediamines, o-/phydroxybenzoic acids, m-/p-phenylenediamines, o-/paminoacetophenones, and so forth. Conclusions Nonpolar solvents such as heptane and toluene selectively solubilize o-HAP from its mixtures with p-HAP. The minute amounts of p-HAP dissolved in the solutions can be selectively sorbed from the organic phase. HAPs interact with the functional group in the resin phase, which results in the formation of a hydrogenbonded complex between HAPs and the tertiary amino group in the resin. The sorption of HAPs was higher from nonpolar solvents as compared to that from polar solvents onto resins. The difference in the sorption behavior is the result of differential solvation of HAPs in the respective solutions with the solvation being better in polar solvents. The tertiary amino groups of a weak ion-


exchange resin interact with relative ease with hydrogen of p-HAP giving specific sorption. p-HAP was selectively removed from its mixture with o-HAP by these basic resins. While the uptake of p-HAP was mainly due to adsorption, the uptake of the o-HAP was mainly due to absorption with very little specific interaction with the resin’s functional sites. Acknowledgment The authors acknowledge the financial support to this work from the Department of Science and Technology (D.S.T.), Government of India (No. III 5(45)/96-ET). Literature Cited (1) Hocking, M. B. 2-Hydroxyacetophenones via Fries arrangements and related reactions. J. Chem. Technol. Biotechnol. 1980, 30, 626. (2) Gaikar, V. G.; Sharma, M. M. Separations through reactions and other strategies. Sep. Purif. Methods 1989, 18 (2), 111. (3) Jagirdar, G. C. Two step separation process for nitrophenols. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 886. (4) Jagirdar, G. C. Separation of nitroanilines. Chem. Ind. 1984, 586.

(5) Anasthas, H. M.; Gaikar, V. G. Adsorptive separations of alkylphenols using ion-exchange resins. React. Funct. Polym. 1999, 39, 227. (6) Anasthas, H. M.; Gaikar, V. G. Adsorption of acetic acid on ion exchange resins in non-aqueous conditions. React. Funct. Polym. 2001, 47, 23. (7) Anasthas, H. M. Reactive Separation Processes. Ph.D. (Technol.) Thesis, University of Mumbai, Mumbai, 2001. (8) Fredenslund, A.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria using UNIFAC; Elsevier: Amsterdam, The Netherlands, 1977. (9) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids; McGraw-Hill Book Co.: New York, 1958. (10) Brune, B. J.; Koehler, J. A.; Smith, P. J.; Payne, G. F. Correlation between adsorption and small molecule hydrogen bonding. Langmuir 1999, 15, 3987. (11) Maity, N.; Payne, G. F.; Chipchowsky, J. L. Adsorptive separations based on the differences in solute-sorbent hydrogen bonding strengths. Ind. Eng. Chem. Res. 1991, 30, 2456.

Received for review May 29, 2001 Revised manuscript received November 20, 2001 Accepted November 20, 2001 IE010471+

p-Hydroxyacetophenones by Selective Solubilization

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